Note: Descriptions are shown in the official language in which they were submitted.
3 l~
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RD-19,818
PXEP~RAT~O~ O~ 8I~CON~OXY-~R~ID~
G~aS83~ ~RO~ $~hOX~OL ~ATED
CO~O~DA~ $I~ICA
Cross Reference to Related Applications: The
subject application relates ~o copending applications for RD-
17,455, Serial No. 359,619; RD-19,110, Ser:ial No. 386,327;
and RD-19,549, Serial NoO 428~711.
~;~
The present invention relates to glass compositions
and in particular to glass compositions comprising silicon,
oxygen, and carbon made from a siloxanol treated colloidal
silica.
Vitreous silica is a refractory glass, however, it
devitrifies at about 1100 C. Devitrification refers to the
transition from the random structures that glasses are made
of to a crystallized structure. Crystallization drastically
reduces one of the predominant attributes of vitreous silicat
i.e., its low thermal expansion, as well as many of its other
desirable properties. As a result, much research has been
directed towards increasing the resistance to devitrification
in silica glass compositions.
Reactions between silicon, carbon, and oxygen have
been studied extensively. Known reactions in a silicon,
carbon, oxygen system include oxygen combining with silicon
to form silica, predominantly as silicon dioxide. At
temperatures in excess of 1100 C silica begins to crystalliæe
to form cristobalite, one of the common mineral forms of
silica. Carbon can react with silicon to form crystalline
silicon carbide or it can react wi~h oxygen to form carbon
monoxide.
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RD-19,818
The thermodynamics of silicon, carbon, and oxygen
reactions is discussed in l'The High-Temperature Oxidation,
Reduction, and Volatilization Reactions of Silicon and Sili-
con Carbide", Gulbransen, E.A., and Jansson, S.A. Oxidation
5 of Metals, Volume 4, Number 3, 1972. The thermodynamic
analysis of Gulbransen et al. shows that at 1200 C silica and
carbon should form gaseous silicon ~onoxide and carbon
monoxide or sol~d silicon ¢arbide, SiC. Howevex, no single
material containing all three elements would be expected to
form. Gulbransen et al. conclude that silica was not
recommended for use in reducing atmospheres above 1125 C due
to the formation of silicon monoxide gas. Also silicon
carbide was not recommended for use in oxygen containing
environments due to oxidation of the silicon carbide.
lS There is a material described as carbon
modified ~itreous silica and herein referred to as "black
glass" where 1-3 percent carbon has been added to silica.
The method for making black glass is disclosed by Smith et
al. in U.S patent 3,378,431. Carbonaceous organics such as
carbowax are added to silica and the mixture is hot pressed
at about 1200 C to form black glass. Smith, C.F., Jr. has
characterized black glass by infrared spectroscopy in "The
Vibrational Spectra of High Purity and Chemically Substituted
Vitreous Silicas", PhD Thesis, Alfred University, Alfred,
N.Y., May 1973. Smith discloses that in addition to
elemental carbon dispersed in the black glass, some carbon is
associated with oxygen in carbonato type groups. The term
carbonato describes a radical having a carbon atom bonded to
three oxygen atoms and having the structure,
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RD-19,818
The mechanical strength of black glass is similar
to the strength of conventional carbon-free silica glass
however black glass has an increased reslstance to
devitrification over conventional silica glass which begins
to devitrify at about llOO C while black glass begins to
devitrify at about 1250 C. The increased thermal stability
of black glass allows it to be used at the higher
temperatures where conventional silica would devitrify.
In a commercially produced continuous silicon car-
bide ceramic fibre sold under the trademark "Nicalon", about
10 percent oxygen is introduced into the fibre to crosslink
it. After crosslinking, the fibres are pyrolized and it is
believed that the oxygen becomes part of the fibre as an
amorphous contaminant, probably in the form of silica. The
degradation behavior of such fibres after heat treatment in
various environments was reported in the article "Thermal
Stability of SiC Fibres (Nicalon~)", Mah, T., et al. r ~ournal
of Material Science, Vol. 19, pp. 1191-1201 ~1984). Mah et
al. found that regardless of the environmental conditions
during heat treatment, the "Nicalon77 fibre strength degraded
when the fibres were subjected to temperatures greater than
1200-C. The fibre degradation was associated with loss of
carbon monoxide from ~he fibres and beta-silicon carbide
grain growth ln the fibr~s.
Ceramic materials generally exhiblt brittle be-
havior as characterized by their high strength and low frac-
ture toughness. Fra~ture toughness is the resistance to
crack propagation in materials. The development of ceramic
composites has been investigated as a way to alleviate the
brittle behavior of ceramics. 7'Nicalon" is an excellent
ceramic fibre but it degrades at temperatures above 1200-C.
Integrating 'INicalon" fibres in a protective ceramic matrix
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RD-19,818
having desirable mechanical properties and capable of with-
standing temperatures substantially higher than 1200-C, would
be one way of forming an improved ceramic composite.
However, from the discussion above, it is apparent that the
properties of known ceramics or glasses, and specifically
those containing silicon, oxygen, and carbon, are degraded
for example by decomposition of silicon carbide or
devitrification of conventional glass.
Therefore, it is an object of this invention to
form a vitreous glass, comprising silicon, oxygen, and carbon
in which a substantial portion of the carbon atoms are
chemically bonded to silicon atoms and the remaining carbon
is elemental carbon dispersed in the glass matrix. Such
glass compositions resist decomposition in oxidizing or
reducing atmospheres and devitrification at temperatures up
to about 1600-C.
Another object of this invention is to provide a
process for forming a glass comprised of silicon~ oxygen, and
carbon by pyrolizing siloxanol treated colloidal silicas.
Still another object of this invention is forming
glass articles from a siloxanol treated colloidal silica.
We have found that siloxanol treated colloidal
silicas can be pyrolized in a non-oxidizing atmosphere to
form unique glass compositionsO Surprisingly, we have found
that iloxanol treated colloidal silicas pyrolized in a non-
oxidizing atmosphere below about 1600 C do not form silica,cristobalite, silicon carbide, carbon monoxide, or mixtures
of silica and elemental carbon.
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RD-19,818
The structurally stable non-crystalline glasses of
this invention are made by pyrolizing a siloxanol treated
colloidal silica to form a glass composition, comprising
silicon, oxygen, and carbon wherein a major portion of the
carbon a~oms are chemlcally bonded to silicon atoms. These
glasses resist crystallization, and decomposition in
oxidizing or reducing atmospheres at temperatures up to about
1600-C. In addition, a major portion of the carbon present
in the glasses of this invention is bonded to silicon with
the remainder present as elemental carbon dispersed within
the glass matrix so that there are no detec~able carbonato
groups.
The carbon-silicon bonds discovered in the glasses
of this invention have heretofore been unknown in silica
glasses. In silica glasses, and specifically in black glass,
carbon has only been known to be present as an unbonded
element in the silica matrix or in carbonato groups where
carbon is bonded with oxygen. The glasses of this invention;
characterized by the chemical bonding described above are
herein referred to as silicon-oxy-carbide glass.
Glasses of this invention are produced from a
siloxanol trea~ed colloidal silica heated in a non-oxidizing
atmosphere to pyrolize the treated silica. As used herein,
the term "non-oxidizing atmosphere" means a substantially
oxygen-free a~mosphere that removes pyrolysis by-products
from the pyrolizing resin without influencing the reactions
occurring during pyrolysis. Examples of non-oxidizing
atmosphsres include a vacuum of less than about 10-4
atmospheres, inert atmospheres like helium, argon, or
nitrogen, and reducing atmospheres, such as hydrogen.
A~ referred to herein, a siloxanol treated
colloidal silica comprises a dispersion of colloidal silica
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RD-19,818
in a partlal condensate of a silanol of the formula RSi(OH)3.
The partial condensate is formed by hydrolysis of an
organotrialkoxysilane in which R is a monovalent hydrocarbon
radical containing from about 1 to 12 carbon atoms such as a
halohydrocarbon/ C(1_12~ alkyl~ C~6_12) aryl, or an ester such
as methacrylate or acrylate; at least 70 wleight percent of
the silanol being methyltrihydroxysilane~ The partial
condensate of the silanol is the siloxanol. The siloxanol
treated colloidal silica is sometimes herein referred to as
treated silica or treated colloidal si}ica.
Pyrolysis of the treated silica forms a silicon-
oxy-carbide glass that is characterized by a continued
sharing of electrons between atoms of silicon, oxygen and
carbon. In silicon-oxy-carbide glass, silicon atoms are
present in up to four polyatomic units. In one unit, herein
referred to as tetraoxysilicon, a silicon atom is bonded to
four oxygen atoms. In a second unitr herein referred to as
monocarbosiloxane, a silicon atom is bonded to three oxygen
atoms and one carbon atom. In a third unit, herein referred
to as dicarbosiloxane~ a silicon atom is bonded to twa oxygen
atoms and two carbon atoms. In a fourth unit, herein re-
ferred to as tetracarbosilicon, a silicon atom is bonded to
four carbon atoms.
~hen a treated silica comprised of the partial
condensate of methyltrimethoxysilane and colloidal silica in
a ratio of about 1.8:1 is pyrolized, a sllicon-oxy-carbide
ylass is formed having a distri~ution of polyatomic units
comprising, about 76 to 86 weight percent tetraoxysilicon,
about 11 to 21 weight percent monocarbosiloxane, and up to
about 8 weight percent dicarbosiloxane, with at least about 3
weight percent of elemental carbon dispersed within the glass
matrix. The polyatomic units are linked primarily by
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RD-19,818
silicon-oxygen bonds with a small and insignificant number of
bonds betwecn carbon and oxygen atoms.
The following description of the invention will be
more easily understood by making reference to the figures
briefly described below.
Figure 1 is a graph showiny the weight lost cluring
pyrolysis of a siloxanol treated colloidal silica.
Figure 2 is a graphical presentation of the
~9Silicon nuclear magnetic resonance spectrum of the silicon-
oxy-carbide glass formed by pyrolizing a siloxanol treated
colloidal silica.
Figure 3 is a graphical presentation of the
29Silicon nuclear magnetic resonance spectrum of "Nicalon"
silicon carbide.
Glasses are generally formed from an extremely
viscous supercooled liquid and possess a polymerized network
stxucture with short-ranse order. The glasses o~ this
invention are not made from supercooled liquids, but they do
possess a network structure with short-range order. Instead
of supercooling a liquid, the glasses of this invention are
formed by pyrolizing a siloxanol treated colloidal silica in
a non-oxidizing atmosphere. Rowever, the glasses of this
invention have the short-range ordering characteristic of
conventional glasses.
The siloxanol treated colloidal silica which can be
used in the practice of the present invention comprises a
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RD-19,818
dispersion of colloidal silica in the partlal condensate of a
silanol of the formula RSi(O~)3,where R is a monovalent
hydrocarbon radical from about 1 to 12 carbon atoms such as a
halohydrocarboll, Ctl-l2) alkyl, C~s_12) aryl, or an ester such
as methacrylate or acrylate; at least 70 weight percent o~
the silanol being methyltrihydroxysilane. Colloidal silica
treated with the partial condensate can be dried to form a
powder, or the treated silica can be diluted in an aliphatic
alcohol-water solution containing about 10 to 50 weight
percent solids, the solids consisting essentially of 10 to 70
weight percent colloidal silica and 30 to 90 weight percent
of the partial condensate, the composition having a pH of
from 3.5 to 8Ø
The treated colloidal silica can be prepared by hy-
drolyzing a trialkoxysilane or a mixture of trialkoxysilanes
of the formula RSi~OR')3, wherein R is as previously defined,
and R' is C~1_g~ alkyl radicals, in the presence of an aqueous
dispersion of colloidal silica.-
Suitable aqueous colloidal silica dispersions
generally have a particular size of from 5 to 150millimicrons in diameter. These silica dispersions are well
known in ~he art and commercially available ones include, for
example, those sold under the trademark of Ludox (DuPont) and
Nalcoag (NALCO Chemical CoO). Such colloidal silicas are
available as both acidic and basic hydrosols. Colloidal
silicas having an average partlcle size of from 5 to 25
millimicrons are preferred. A particularly preferred one for
the purposes herein is known as Ludox AS-40, sold by the
DuPont Company.
In preparing the treated ~olloidal silica
composition, the aqueou colloidal silica dispersion is added
to an alkyltrialkoxysilanP or aryltrialkoxysilane which may
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RD-19,818
contain a buffexing agent such as acetic acid, alternatively
alkyltriacetoxysilane may be used in place of the
alkoxysilanes and acid. If desired, small amounts of dialkyl
dialkoxysilane also can be utilized in the reaction mixture.
The temperature of the reaction mixture is maintained at
about 20 C. to about 40 C. and pre~erably below 25 C. It has
been found that in about six to eight hours sufficient
trialkoxysilane has reacted to redu~e the initial two-phase
liquid mixture to a single liquid phase in which the silica
is dispersed.
In general~ the hydrolysis reaction is allo~ed to
continue for a total of about 12 hours to 48 hours, depending
upon the desired viscosity of the final product. The more
time the hydrolysis reaction is permitted to continue, the
higher will be the viscosity of the colloidal silica
dispersion.
After hydrolysis has been completed, the solids
content is adjusted by the addition of alcohol, preferably
isopropanol, to the colloidal silica dispersion. Other
suitablP alcohols for this purpose include lower aliphatic
alcohols such as methanol, ethanol, isobutanol, isopropanol,
n-butyl alcohol and t-butyl alcohol or mixtures thereof.
When it is desirable to use a treated colloidal silica in
which the partial ~ondensate is in solutionl the solvent
system can contain from about 20 to 75 weight percent
alcohol.
The pH of the resultant reacted composition is in
the range of from about 3.5 to 8.0j preferably from about 6.6
to about 7.3 or from 3.8 ~Q 5.7. If necessary, a dilute
base, such as ammonium hydroxide, or weak acid, such as
acetic acid, may be added to the composition to adjust the pH
to the desired range.
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RD-19,818
The acid is used to buf~er the basicity of the
initial two liquid phase reaction mixture and thereby also
~emper the hydrolysis rate. ~lacial acetic acid as well as
other acids such as organic acids like propionic, butyr~c,
citric, ben20ic, formic, oxalic and the like may be used.
Alkyltriacetoxy silanes wherein the alkyl group contains from
1-6 carbon atoms can be used, alkyl gxoups having from 1 to 3
carbon atoms being preferred. ~ethyltxiacetoxysilane is most
pre~erred.
The silanetriols, RSi(OH)3, hereinbefore mentioned,
are formed as a result of the hydrolysis of the corresponding
trialkoxysilanes with the aqueous medium, i.e., the aqueous
dispersion of colloidal silica. Exemplary trialkoxysilanes
ar~ those containing methoxy, ethoxy, isopropoxy and n-butoxy
substituents which, upon hydrolysis form the silanetriol and
the corresponding alcohol. If a mixture of trialkoxysilanes
is employed, a mixture of different silanetriols, as well as
different alcohols, is produced. Upon the production of the
silanetriol or mixtures of silanetriols in the basic aqueous
medium, condensation of the hydroxyl suhstituents to form
--S i--O--S i--
bonding occurs. This condensation takes place over a period
of time and is not an exhausting condensation but rather the
siloxane retains an appreclable quantity o~ silicon-bonded
hydroxyl groups which render the polymer soluble in the
alcohol-water cosolvent. It is believed that this soluble
partial condensate can be characterized aq a siloxanol
polymer having at least one silicon~bonded hydroxyl group per
e~ery three
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RD-19,81R
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units.
The non-volatile solids portion of the treated
colloidal silica herein is a mixture of co]Lloidal silica and
the partial condensate or siloxanol, of a silanol. The major
portion or all of the par~ial condensate or s~loxanol is ob-
tained from the condensation o~ me~hyltrihydroxysilane and,
depending upon the input of ingredients ~o the hydrolysis
reaction, minor portions of partial condensate can be
obtained, for example, from the condensation of
methyltrihydroxysilane with e~hyltrihydroxysilane, or
propyltrihydroxysilane; methyltrihydroxysîlane wi~h
C6HsSi(OH)3, or mixtures o~ the foregoing. It is preferred to
use all methyltrimethoxysilane, thus producing all methylsi-
lanetriol, in preparing the treated colloidal silica compo-
sitions which can be dried to form a powder or dissolved in a
solvent so the partial condensate is present in an amount of
from about 55 to 75 weight percent of the total solids in a
cosolvent of alcohol and water~ the alcohol comprising from
about 50~ to 95% by weight of the cosolvent.
Silicon-oxy-carbide glass is formed by pyrolysis of
siloxanol treated colloidal silica in a non-oxidizing
atmosphere at tempera~ures between about 900 C and 1600 C.
Preferably the heating rate during pyrolysis is controlled ~o
allow the pyrolysis by-products to escape without leaving
voids or bubbles in the silicon-oxy-carbide glass.
Preferably heating rates of less than l C per minute are
used. Durlng pyrolysi~, by products evolve and cause a
weight loss as the treated iliea densifies. Although the
pyrolizing treated silica experiences a weight loss, the
density of the pyrolizing treated silica is increasing due to
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RD-19,818
a reduction in volume of the pyrolizing treated silica. The
pyrolysis reactions are essentially completed when a
substantially constant weight was achievecl in the pyroli7ing
treated silica. A substantially constant weight is generally
achieved at about 900 C to 1250 C. Further densification of
the pyrolizing treated silica may occur after weight loss has
ended~ if heating is continued. Therefore, i.t may be
desirable to stop heating and pyrolizing of the treated
silica after it has completely densified, or in other words,
stops redu~ing in volume. Weight loss during pyrolysis of
one treated silica was determined to be about 14 percent.
The glasses of this invention resist devitri-
fication, and remain structurally stable at temperatures up
to at least 1600-C. The term "structurally stable" refers to
a bulk material that retains essentially the same micro-
structure from room temperature up to about 1600 C. The
formation of small crystallized areas up to about 100
angstroms in an otherwise amorphous matrix have substantially
no adverse or deleterious effect on the properties of the
bulk material. Therefore, structurally stable glasses of the
present invention are essentially amorphous but may contain
small crystallized areas of, for example, graphite, cristo-
balite, or silicon carbide witAin the glass, or display minor
amounts of cristobalite on the surfaces of the glass.
Articles of silicon-oxy-carbide glass can be formed
by pulveri~ing the pyrolized treated silica into a powder
using grinding mills well known in the art. The silicon-oxy-
carbide powder is then consolidated by hot pressing to form
an article. One method for hot pressing is to apply a
30 uniaxial pressure of at least about 5 ksi at about 1550C to
1600 C to the powder. The unit ksi is kips per square inch;
the equivalent of 1000 pounds per square inch. Such
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RD-19,818
pressures and temperatures are sufficient to ~orm a densified
article.
Shaped articles can also be formed directly from
the treated silic~. First, the treated si.lica is put in
solution in a solvent such as isopropanol and then cast into
a desired shape. Illustrative of the solv~nts that have been
found suitable for placing the treated sil:ica in solution are
lower aliphatic alcohols such as methanol, ethanol,
isobutanol, n-butyl alcohol and t-butyl alcohol or mixt~res
thereof, or preferably isopropanol. Treated silicas can be
dissolved in about 20 to 75 weight percent soIvent.
The cast treated silica is dried at room
temperature and slowly pyrolized in a non-oxidizing at-
mosphere as described herein. Pyrolysis is performed at a
low rate of heating that avoids formation of voids and
bubbles as gases evolve and cause a weight loss in the
treated silica. When the weight of the pyrolizin~ treated
silica stabilizes, pyrolysis is complete. Alternatively,
the treated silica, which is normally in the form of a
powder, can be shaped by hot pressing.
The treated silica in isopropanol solution can also
be drawn into ribres. The treated silica solution is treated
with a base such as ammonium hydroxide to increase the
viscosity to a point where a solid object can be dipped into
the solution and withdrawn, pulling a strand of the treated
silica from the solution. Fibres can then be drawn or pulled
from the treated silica solution by such dipping processes.
Alternatlvely, the treated silica solution can be drawn into
a teflon tube with a slight vacuum. As the isopropanol
evaporates and the ~reated silica solution increases in
viscosity, khe fibre shrinks and is pushed out of the tube.
Fibres are strengthened for easier handling by heating them
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RD-19,818
to about 50 C. The fibres are then pyroliz.ed in a non
oxidizing atmosphere or a vacuum as described above.
Ceramic composites can be formed having ceramic
fibres in a matrix of silicon-oxy-carhide ~lass and ceramic
filler. Trea~ed silica is put in solution in a solvent, and
ceramic particle~ are dispersed in the solution to form an
infiltrant slurry. The particulate ceramic filler con~rols
shrinkage of the composite ma~rix during pyrolysis and can be
chosen so the matrix is compatible with the fibre
reinforcement to be used. Some examples of c~ramic fillers
are powdered silicon carbide, diatomaceous earth and the
2Si02-3A1203 aluminosilica~e referred to as mullite.
A ceramic fibre or fibres, or a cloth of the fibres
is drawn through an agitated bath of the infiltrant slurry.
Some examples of ceramic fibres are carbon fibre, silicon
carbide fibre and alumino-boro-silicate fibres. The im-
pregnated fihre is then shaped and dried to allow evaporation
of the solvent. One shaping method includes winding an
impregnated fibre spirally on a drum to form a panel. Layers
of the f.i~re can be consolidated ~hrough the application of
heat and pressure to form a continuous treated silica matrix
surrounding the ceramic fibres. The composite is then
pyrolized in a non~oxidizing atmosphere or a vacuum as
described above. The treated silica densifies into a
substantially amorphous silicon-oxy-carbide glass that binds
the ceramic filler, thus forming a continuous matrix around
the fibres. Depending on the pyrolysis temperature used, the
ceramic filler may be dispersed, partially sintered or fully
sintered within the glass.
Optionally, the ceramic composite can be re-in-
fil~rated with the infiltrant slurry to reduce porosity in
the composite. The composite is placed in the re-infiltrant
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RD-19,818
solution while in a vacuum. Pressure is applied to the
solution to force the solu~ion into the pores of the
composite. After re-infiltrating, the sol~ent is allowed to
evaporate and the re-infiltrated composite is pyrolized in a
non-oxidizing atmosphere or vacuum as described above. Re-
infiltration and pyrolysis can be repeated as often as needed
to achieve the desired degree of density in ~he matri~.
The matrix of amorphous silicon-oxy-carbide glass
binding a ceramic filler surrounds and protects the ceramic
fibres from decomposition in oxidizing and reducing atmo-
spheres at temperatures up to about 160Q-C. It was found
that the inert natuxe of silicon-oxy-carbide glass readily
accepts ceramic fibres without reacting with them and
degrading their properties. As a result, silicon-oxy-carbide
glass containing appropriate ceramic fillers can be used as a
matrix material for many known ceramic fibres.
The following examples are offered to further il-
lustrate the silicon-oxy-carbide glass of this invention and
methods for producing the glass and glass articles.
The siloxanol treated colloidal silica composition
comprising the partial condensate derived from
methyltrimethoxysilane, and colloidal silica in a weight
ratio of about 1.8:1 was pyrolized while weight loss from the
treated silica was measured by thermal gravimetric analysis.
Thermal grav metric analysis is a method for measuring weight
loss from a sample while it is being heated. The material
was pyrolized in a hydrogen atmosphere by heating at a rate
of lO C/minute to a temperature of 1250-C. The measured
weight loss for the silicon-oxy-carbide glass formed after
pyrolysis was about 14 percent.
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RD-19,818
The weight loss data from pyrolysis of the treated
silica, i5 presented in ~.he graph of Figure 1. In the graph
of Figure 1, the percent weight loss in the sample is plotted
on the ordinate while the increase in heat:lng temperature is
plotted on the abscissa. The graph of Figure 1 shows weight
loss is essentially complete in the sample at about 900 C.
Substantially no evidence of crystalliza~ion was found by x-
ray diffraction of the pyrolizPd material.
The composition of different glasses can be broadly
defined by referring to the amount of each element in the
glass. However, it is the shor~-range ordering in glasses
that give them their differen~ properties. Therefore, b.y
characterizing the short-range ordering in glasses different
glass compositions can be defined with respect to properties.
The short range ordering of the silicon-oxy-carbide glass
prepared in Example 1 is determined by defining the
percentage of each of the polyatomic units;
monocarbosiloxane, dicarbosiloxane, and tetraoxysilicon that
are present in the glass.
The 29Silicon solid state nuclear magnetic
resonance spectrum of the silicon-oxy-carbide glass prepared
above was recorded and is presented in Figure 2. Figure 3 is
the 29Silicon nuclear magnetic resonance spectrum from a
sample of "Nicalon" silicon carbide fibre. On the ordinate of
Figures 2 and 3 is plott~d the intensity of radiation
measured from the excited sample, and on the abscissa is
plotted the parts per million (ppm~ in chemical shift from a
tetramethyl silicon standard that fixes the zero point on the
abscissa. The chemical shift in ppm are known for many
polyatomic units, for example ~etraoxysilicon,
dicarbosiloxane and monocarbosiloxane are shown in; "NMR
Basic Principles and Pro~ress 29Si-NMR Spetroscopic Results",
,.~ .
-17-
llL15
RD-19,818
Editors P. Diehl, R. Kosfeld, Springer Verlag Berlin
Heidelberg 1981 at pp. 186, 184 and 178. Therefore, each
peak in Figures 2 and 3 defines the short-range ordering of
specific silicon polyatomic units.
The spectra of the silicon-oxy-carbide glass in
Figure 2 contains peaks labeled 1 through 3. Peak 1 is
dicarbosiloxane, peak 2 is ~onocarbosiloxane, and peak 3 is
tetraoxysilicon. By integrating the area under each peak,
the fraction of each of these polyatomic units that is
present in the glass can be determined. A correction for
background interference was made to the spectra in Figures 2
and 3 before determining the integrated area under each peak.
The integrated area under each peak in Figure 2
reveals a composition for the silicon-oxy-carbide glass
prepared as described above comprising, up to about 8 weight
percent dicarbosiloxane, about 11 to 21 weight percent mono-
carbosiloxane, and about 76 to 86 wei~ht percent
tetraoxysilicon. ~nalysis of the nuclear magnetic resonance
spectra and the black appearance of the glass indicates that
at least about 3 weight percent of elemental carbon is
dispersed in ~he glass either atomically or in small
clusters.
Treated colloidal silicas having various ratios of
siloxanol to colloidal silica can be pyroliz-ed to form
silicon-oxy-carbide glasses. It is expected that treated
colloidal silicas having a ratio of siloxanol to colloidal
silica that is greater than 1.8:1 will have a decreased
tetraoxysilicon content that is in the lower part of the 76
to 86 percent range described above, or lower. Conversely
treated colloidal silicas having a ratio of siloxanol to
colloidal silica that is less than 1.8:1 are expected to have
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RD-19,818
an increased tetrao~ysilicon content that is in the upper
part of the 76 to 86 percent range or higher.
The spectra in Figure 2 can be compared to the
silicon carbide spectra in FIG. 3 measured from a 'INicalon"
silicon carbide fibre sample. The composi~ion for "Nicalon"
in FIG. 3, in weight percent, is about 68 percent silicon
carbide/ about 8 percent dicarbosiloxane~ about 17 percent
monocarbosiloxane, and about 7 percènt tetraoxysilicon. From
the spectra in FIG. 3, it can be seen that "Nicalon" fibres
are comprised principally of silicon carbide with minor
amounts of dicarbosiloxane, monocarbosiloxane, and
tetraoxysilicon. In contrast, the spectra of FIG. 2 shows
that silicon-oxy-carbid~ glass is comprised of substantial
amounts of dicarbosiloxane, monocarbosiloxane, and
tetraoxysilicon. This unique short range ordering of
silicon-oxy-carbide glass that bonds carbon to silicon in a
heretofore unknown manner in glasses, provides the increased
devitrification and decomposition resistance and
characterizes the glasses of this invention.
The composition of the silicon-oxy-carbide glass
sample and Nicalon sample can also be described by referring
to the mole percent of each polyatomic unit. Table I helow
provides the conversion be~ween mole percent and weight
percent for each of these compositions.
$ ~ ~
~19--
l~LL~
RD-19,818
~.hlQL~
Sillc~n-Oxy-Carblde Gl~s "Nicalon"
_Sl~
,,,,,_ .
~etraoxy3ilicon 76-86 74 84 7 5
Monocarbo lloxane 11-21 13-23 17 13
Dicarb~iloxaneup to 8 up to 8 ~ 7
Tetracarbo~ilicon - ~68 75
The mole percent gives the percentage of each
polyatomic unit in the samples on a molecular basis. The
percentage of the silicon atoms in the samples that is bonded
to oxygen or carbon can then be determined using the mole
percent. The silicon-oxy-carbide glass sample from Example
1, has about 16 to 26 percent of the silicon atoms in the
glass bonded to at least an individual carbon atom. The
"Nicalon" silicon carbide sample had about 90 to 100 percent
of the silicon atoms in the silicon carbide sample bonded to
carbon.
